For controlling an AC rotary electric machine, feedback control and feedforward control are performed generally based on detection values of various physical quantities or calculation values and the like, which are calculated from the detection values. For controlling a motor, which is an AC rotary electric machine used in an electric power steering system of a vehicle to generate a steering assist torque, for example, an output torque of the motor is commanded based on a steering torque detected by a torque sensor and electric power supply to the motor is controlled to generate the command torque.
In this exemplary case, the actual torque detection value often deviates from a reference value because of an individual difference of each torque sensor and variations of a hardware circuit, to which the torque detection signal is inputted. According to an electric power steering system disclosed in JP 3933536 (patent document), driving of a motor is controlled by storing a correction value as correction value information for correcting a deviation of a torque detection value of a torque sensor from a reference value and correcting the torque detection value with the stored correction value information. When the correction value information is abnormal, the torque detection value is corrected with a predetermined value.
In the apparatus according to patent document, only the torque detection value is corrected. However, the physical quantities detected for controlling the motor include a rotation position of the motor and the like in addition to the torque detected by the torque sensor. Specifically, as the rotation position, rotation position information such as a mechanical angle, an electrical angle, an electrical angular velocity and a rotation speed is calculated based on a detection signal of a rotation position sensor and a control operation is performed by using these calculated values.
The detection signal of the rotation position sensor includes an initial error such as an assembling error of each rotation position sensor to a motor. The initial error may cause, depending on its value, the motor to generate torque of a different polarity from a command value. That is, the torque has a positive or negative polarity, which is opposite to that of the command. As a result, in the case that the motor is used in the electric power steering system, the motor is driven to generate assist torque in a direction opposite to the direction of steering operation of a driver. This gives feeling of strangeness to the driver.